ANN ARBOR, Mich.For the second straight month, the average fuel economy of all new vehicles sold in the United States remained the same, according to a researcher at the University of Michigan Transportation Research Institute.

The average fuel economy of vehicles purchased in September was 22.1 mpg, unchanged from August.

According to Michael Sivak, research professor and head of UMTRI's Human Factors Group, average fuel economy of all new vehicles bought last month is at its lowest level in the past year. In fact, average fuel economy for new vehicle purchases has trended downward since it hit a high of 23.0 mpg last March.

"This decrease is likely related to a slight drop in gas prices since spring, because people tend to buy more fuel-efficient vehicles when gas prices go up," Sivak said.

Despite being at a 12-month low, average fuel economy for new vehicles sold is still 2 mpg better than that just four years ago.

"Our analysis suggests this long-term improvement in vehicle fuel economy is associated with both gas prices and unemployment being higher now than four years ago," Sivak said.

From now on, Sivak and his colleague Brandon Schoettle will release monthly updates of the average fuel economy of vehicles purchased. They will also continue issuing monthly updates of their new national Eco-Driving Index (EDI), which estimates the average monthly amount of greenhouse gases produced by an individual U.S. driver who purchased a new vehicle that month. EDI takes into account both the fuel economy of the vehicle and the distance driven.

EDI currently shows that emissions of greenhouse gases per driver of newly purchased vehicles are down 14 percent since late 2007.